1. Field of the Invention
This invention relates to floating offshore platforms. More particularly, it relates to offshore platforms having steel catenary risers attached thereto.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A semi-submersible is an offshore, floating unit, with its deck supported by columns to enable the unit to become almost transparent for waves and provide favorable motion behavior. The unit stays on location using dynamic positioning and/or is anchored by means of catenary mooring lines terminating in piles or anchors.
A tension leg platform is a floating production platform positioned and stabilized by at least three separated, vertical tendons anchored to the seabed. The tendons are tensioned using the buoyancy of the underwater hull of the platform. Subjected to wave, wind and current action, the platform moves sideways, but remains horizontal due to the parallel actions of the tendons. The vertical motion (heave) is eliminated and the facility is therefore suitable for surface completion of the wells.
Both tension leg platforms (TLP's) and semi-submersible floating vessels (“semis”) may be used for offshore drilling and production operations.
A Steel Catenary Riser (SCR) is a steel pipe hung in a catenary configuration from a floating vessel in deep water (such as a TLP or semi) to transmit flow to or from the sea floor.
Attachment points on the floating vessel for Steel Catenary Risers (“riser porches”) are typically located at or near the outer perimeter of tension leg and semisubmersible platforms, on the outboard side shell of the base structure pontoons. As such, they are susceptible to the global motions of the platforms, creating long-term fatigue damage which is often the governing criteria in the design of SCR components. While global heave, surge and sway translate directly into relative linear motions at the SCR porches, the net displacements may be amplified by rotational pitch and roll, and to a lesser extent, by yaw motions, based on the radial distance with respect to the center(s) of global rotation. Reducing this distance may lower the roll/pitch-induced displacement, and thereby reduce cumulative fatigue damage in the SCR components. Thus, the closer the SCR porch can be attached to the center of global roll and pitch rotation of the supporting vessel, the longer the fatigue life may be extended.
Cost comparisons indicate a significant economic advantage in the dry-tree platform, on-board drilling approach with respect to comparable tender drilling, wet-tree designs. However, in order to produce a functional, efficient Top Tensioned Riser system, while maintaining stroke within the range of existing hardware, some form of riser keel guide is often required. This applies not only to vessels having a battered-column hull, (e.g., as disclosed in U.S. Pat. No. 7,462,000 to Leverette et al. entitled: “Battered column tension leg platform”) but to conventional, vertical-column semi-submersibles and TLPs. The keel guide is typically designed to carry only the dominant lateral loads of the riser array, as well as some consequential friction and inertial loading. However, the deep box-section guide structure described herein may easily accommodate the additional loading of SCR porches, located as far inboard as required to reduce the amount of accumulated fatigue damage to acceptable levels. While it was designed to span a much greater distance than in the typical platform, and modified to effect a change in platform dynamics, the basic design of the structure is easily adaptable to all Semi-Submersible and TLP designs. The concept may thus provide a level of dynamic loading reduction, and thereby comparable SCR fatigue life extension for all applicable platform types.